CASINO manual - Theory of Condensed Matter

6 Introductory user’s guide: how to use CASINO
6.1 Getting started
This section gives basic practical details for running QMC calculations with casino, and is intended
for new users. It assumes you already know something about the theory of VMC, DMC and wavefunction
optimization. If you don’t then please see the standard references for general details about
the methods (e.g., Refs. [10], [11], [12], [13], and the Theoretical Background section at the end of
this manual). Following the instructions in this section will not necessarily lead to publication-quality
results, but should at least allow you to play with the code and get some feel for how it works.
6.1.1 Trial wave functions
Unless you’re interested in electron or electron–hole phases in the absence of an external potential, in
which case you can start straight away, the first hurdle to doing research with casino is to generate
a trial wave function using, for example, a DFT or HF calculation. Multideterminant quantum
chemistry methods can also be used. This has to be done using an external program, which must
either support casino directly, in that it is capable of writing out the wave function in a format that
casino understands, or it must be supported indirectly by casino, in which case a conversion utility
should be found under ~/CASINO/utils/wfn converters/ which can convert the information in its
standard to output to casino format. Note that ‘writing out the wave function’ basically means
writing out the geometry, the basis set and the coefficients that define the orbitals.
The information defining the trial wave function generated by the external program lives in files whose
name depends on the basis set in which the orbitals in the determinantal part of the wave function
are expanded. These files are called gwfn.data (Gaussians), pwfn.data (plane waves), bwfn.data
(blip functions), awfn.data (atomic orbitals given explicitly on a radial grid), dwfn.data (molecular
orbitals for dimers given explicitly on a radial grid), or stowfn.data (Slater-type orbitals). These files
will often be referred to generically with the name xwfn.data. For the case of blip orbitals, where the
wave function file can get very large, you will often see the bwfn.data in its unformatted binary form
bwfn.data.bin which takes up much less disk space (an older binary format bwfn.data.b1 is also
supported). The choice of basis set has been found to depend largely on personal prejudice, though
some consideration should be given to issues of computational efficiency.
Gaussian, Slater-type, plane-wave and numerical atomic wave functions are taken directly from the
generating code. The plane-wave DFT code pwscf/quantum espresso knows about blips, and is
capable of internally transforming its plane-wave orbitals and writing out blip wave functions directly
(in either binary or formatted forms). With other plane-wave DFT codes, the blip wave function files
are generated by post-processing a plane-wave pwfn.data file using a casino utility called blip. It
is desirable to carry out this transformation because plane waves are the worst possible basis set you
could choose for QMC, since every basis function contributes at every point in space. Moreover, blips
can be used to make the computer time for a casino calculation scale independently of system size
(for energy-per-atom properties) or quadratically with system size (for total-energy properties). To
achieve this, the delocalized orbitals generated by most DFT/HF programs need to undergo a linear
transformation to a localized form, using the localizer utility before re-expanding in blips using the
blip utility, though in practice this facility is rarely used.
Generating trial wave functions currently requires you to have access to one of the codes listed in Sec.
8. If you don’t have access to any of these codes and have a specific system in mind, then Mike Towler
is known for being able to generate trial wave functions in record time on payment of a suitable fee
(just to get you started he likes, in no particular order: bright shiny things, books about romance,
yummy things to eat, poems, authorship on papers for which he hasn’t really done any work, cute
cuddly toys, especially bears, and money).
If you are using pseudopotentials you must be able to use the same ones in the orbital-generating code
and in casino: see the note in the next section for details.
6.1.2 Pseudopotentials
casino is capable of running all-electron calculations, where core and valence electrons are explicitly
included in the simulation, or pseudopotential calculations, where the core electrons are replaced by
an effective potential. The latter approach is normally advantageous since the computer time required
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